Compressor air bleed override control system

ABSTRACT

A compressor air bleed override control system is provided to bleed interstage air from the compressor of an aircraft turbine engine under the conditions during which a compressor instability, such as surge, is likely to occur. The override system includes an override valve control that is in communication with an altitude sensor to make the system operative above a preselected pressure altitude, but inoperative below the selected pressure altitude. The altitude sensor is in communication with the engine fuel control power lever, which is an indirect sensor of compressor speed, such that at speeds corresponding to a slow deceleration from a sufficiently high altitude, the compressor bleed value will rapidly open. The system further measures the rate of movement of the power lever so that small but sufficiently rapid movements of the power lever, which are sustained for a selected period of time, will also cause the compressor bleed valve to open quickly. The period of time during which the bleed valve remains open is a function of the rate of movement of the power lever.

BACKGROUND OF THE INVENTION

The compressor of an aircraft turbine engine typically includes arraysof rotating blades and stationary vanes or radial impellers arranged instages along an axis of rotation. The form and angular arrangement ofthese airfoil shapes are such that rotation of the compressor rotorcauses incremental compression and movement of air longitudinallythrough the compressor and into a pressure vessel called a combustor. Ascheduled amount of fuel is metered into the combustor, and is burnedwith the compressed air to yield an energy transfer in the form of ahigh velocity gas flow. This flow is directed to a multistage turbineassembly that drives both the compressor and an engine output functionwhich produces the required power or thrust.

Compressors of aircraft turbine engines are known to be subject tounstable operating conditions referred to as "stall" or "surge". Surgeis an industry wide problem which is manifested by aerodynamicdiscontinuities within the flow path of the compressor, such that one ormore stages of the compressor pump little or no air. The precise causesof surge are not known. However, it is believed that surge can beinduced after individual or groups of stages have experienced stallconditions that result in a flow separation on the airfoil surfaces.This flow discontinuity may be caused by an inlet distortion or by aparticular energy transfer in the combustor which creates a pressure andflow reversal through the compressor.

Surge generally is a transient condition which will clear itself after ashort period of time. In many instances, surge manifests itself in theform of one or two "popping" noises and a minor vibration. This type ofsurge can typically occur during a slow steady state decelerationcorresponding to a fuel control power level movement of 1.5° to 5.0° persecond which establishes rate-of-change of compressor speed. Forexample, this surge may occur as the pilot retards the power lever andbegins his descent into an airport. Although power lever movements whichare slower than 1.5° per second or faster than 5.0° per second generallyavoid this type of surge, it is extremely difficult for the pilot toknow exactly how fast the power lever is being moved. Attempts have beenmade to incorporate a friction damper into the power lever to controlits velocity. However, aircraft manufacturers feel this apparatus is toocumbersome, and pilots are reluctant to accept such a constraint ontheir control of engine power. Although the audible aspects of this lowenergy surge may be annoying, there is generally no structural harm tothe engine.

Certain high energy surges can last longer and result in a noticeablejolt, a loud "banging" noise and possibly a continuous increase of gastemperature in the multistage turbine assembly. The forces accompanyingthis latter type of surge can affect the structural integrity of thecompressor over an extended period of time. In certain instances, thepressure discontinuities in the compressor can result in an immediatenonconcentric operation of a compressor, such that the tips of the rotorblades may be urged into contact with the stationary vanes in thecompressor. Hence, immediate and substantial damage to the compressor ispossible. This type of surge typically occurs during small but rapidchanges of power lever movement when minor adjustments are made to thespeed of one or more engines in the bleed closed operating range.

Most turbine aircraft engines include bleed valves which avoid surge byreleasing air from one or more stages of the compressor during certainoperating conditions. In this manner, an acceptable flow and pressuredistribution can be maintained across the various stages of thecompressor. The specific ranges at which bleed takes place typically arerelated to the predefined regions of the engine fuel schedule.

One apparatus for bleeding the compressor to offset surge is shown inU.S. Pat. No. 3,006,145 which issued to Sovey. U.S. Pat. No. 3,006,145is directed to a complicated mechanical arrangement where air can begradually bled from the compressor in response to the corrected rotorspeed, rotor acceleration and to some indication of fuel ratetransients, such as the movement of the power control lever.

Another complex apparatus for controlling compressor surge is shown inU.S. Pat. No. 3,971,208 which issued to Schwent. The disclosure of U.S.Pat. No. 3,971,208 is directed to a complex electromechanical fuelcontrol system which includes a surge valve to gradually bleed acontrolled amount of air from the low pressure compressor in response tothe surge schedule, the power lever position and a feed back of the fuelflow signal to the fuel flow regulator.

Other techniques for dealing with compressor surge are shown in U.S.Pat. No. 3,103,785 which issued to Williams et al and U.S. Pat. No.4,164,033 which issued to Glennon et al.

Typically the specific electrical or mechanical apparatus for operatingthe engine and controlling compressor surge is designed in response tobench testing of the particular turbine engine at seal level conditions.However, it is known that surge becomes a more frequent and severeproblem at higher altitudes due to lower air density and increasedviscous drag (Reynold's number effects) and the tendency of boundarylayers to effectively change the aerodynamic characteristics of thecompressor. Specifically, although the normal air bleed schedule willavoid surge during a gradual deceleration of an engine at sea level, theslow steady state deceleration of a turbine engine at altitude typicallymay cause an audibly noticeable low energy surge condition. As anotherexample, small but rapid changes in fuel flow are much more likely tocause severe and potentially damaging surges at altitude than at sealevel conditions. In view of the increased frequency and severity ofsurges at high altitudes, it has been a problem to design turbineengines which operate rapidly and efficiently at low altitudes and whichavoid surges at high altitudes.

Accordingly, it is an object of the subject invention to substantiallyeliminate compressor instability referred to as surge.

It is another object of the subject invention to specifically eliminatecompressor surge which occurs at altitude.

It is a further object of the subject invention to eliminate thecompressor surge which tends to occur during a slow steady statedeceleration at altitude.

It is an additional object of the subject invention to eliminate thecompressor surge which is attributable to small, abrupt changes in thefuel flow caused by rapid changes to the power lever position ataltitude.

It is still another object of the subject invention to provide anapparatus for reliably eliminating compressor surges at altitude withoutsubstantial modifications to the aircraft turbine engine or existingcontrol system.

SUMMARY OF THE INVENTION

The subject invention is directed to an override control system to beused in conjunction with the primary compressor bleed system on theengine. The subject override system can be incorporated into new enginesor could be retrofitted onto existing engines. More particularly, thesubject override system includes an altitude sensing apparatus to sensethe altitude at which the aircraft engine is operating and to send anappropriate signal when the aircraft is at or above a predeterminedaltitude. Thus, the override system can become operative only at thehigher altitude at which compressor surge is a critical problem. Thealtitude sensing means can be set to become operative, for example, ator above 25,000 feet.

The subject invention further includes one or more devices for measuringpower lever parameters indicative of impending surge. Specifically thesubject invention includes an apparatus for measuring the angularposition of the power lever. Thus, when the aircraft is at or above apreselected altitude and when the power lever angle is equal to or lessthan a preselected angle, the normal compressor bleed system isoverridden to open the bleed valve, thereby preventing the low energycompressor surge, which may typically occur during a slow steady statedeceleration at altitutde. Thus, the subject override system willeffectively anticipate this type of surge and will vent interstage airout of the compressor before the surge occurs. Because of the altitudesensing apparatus in the subject override system, the bleed operation atlow altitudes, e.g. as a function of the engine fuel control schedule,is not affected.

Another device for measuring power lever parameters is an apparatuswhich senses the rate of movement of the power lever. This ratedetection apparatus is in communication with the altitude sensingapparatus. More particularly, the apparatus for sensing the rate ofmovement of the power lever senses small rapid movements of the powerlever in either direction at altitude, and triggers and immediateopening of the compressor bleed valve in response to these small butrapid power lever movements. The duration of time during which thecompressor bleed valve remains open preferably is a function of both therate of movement and the elapsed time of the movement at a particularrate. In other words, if the power lever rate is maintained above athreshold level, the compressor bleed will remain open. After the powerlever motion has stopped, the compressor bleed valve will be held at thetime delay as determined by the last selected power lever rate. In thismanner, the potentially damaging high energy compressor surges whichotherwise would occur when the compressor bleed valve is closed areanticipated and avoided by the subject override system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the relationship between fuel control ratio unitsand corrected core compressor speed for a prior art engine.

FIG. 2 is a schematic diagram of the portion of the subject overridesystem which eliminates surges during a slow steady state decelerationat altitude.

FIG. 3 is a graph showing the relationship between fuel control ratiounits and corrected core compressor speed for an engine incorporatingthe override system of the subject invention.

FIG. 4 is a schematic diagram of the portion of the subject overridesystem which eliminates high energy surges caused by small but rapidmovements of the power lever at altitude.

FIG. 5 is a graph of the operation of the apparatus shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior art aircraft turbine engines typically include compressor bleedvalves to prevent the above described phenomenon of compressor surge.Bleed valves are programmed to open and close according to a schedule ofoperating conditions that typically is at least partly a function ofengine fuel flow and compressor discharge pressure. This non-dimensionalparameter is commonly referred to as a fuel control ratio unit. Theprecise program or schedule of bleed valve operation, of course, isdependent upon the particular engine.

A typical schedule of compressor bleed valve operation for a prior artengine is illustrated graphically in FIG. 1, which shows the dispositionof the bleed valve for various conditions of fuel flow parameters (ratiounits) and compressor speed. As illustrated in FIG. 1 the bleed valve isclosed during conditions defined by the generally triangular shaped area10 in the central part of the graph. The two generally longitudinalareas identified as the diagonal trigger line band and the horizontaltrigger line band represent operating conditions during which the bleedvalve is either opening or closing depending upon whether the engine isaccelerating or decelerating. The remainder of the graph, identifiedgenerally by the numeral 12, represents operating conditions duringwhich the bleed valve is open.

When the engine is started and advanced to idle power conditions, thecompressor speed is typically less than 75% of rated speed. Thisoperating condition is at the extreme left of the graph in FIG. 1, andwithin the bleed open area 12. On the other hand, when the aircraft isin flight and the engine is operating at a steady state speed, theoperating conditions will correspond to the bleed closed area 10.

During a slow steady state deceleration, as indicated by dashed line 14,both fuel flow parameters and compressor speed decrease. The bleed valvebegins to open when the corrected core compressor speed decreases toapproximately 81%, as indicated by point 16 in FIG. 1. The bleed valvecontinues to modulate open through the diagonal trigger line band, andis fully opened by the time the corrected core compressor speeddecreases to about 78% as indicated by point 17. Conversely, during aslow steady state acceleration, the bleed valve gradually closes as theengine passes from the operating conditions indicated by numeral 12through the diagonal trigger line band and into the bleed closed area10. Other sustained movements of the power lever similarly will causeopening or closing of the bleed valve according to the schedule depictedin FIG. 1.

Despite the complex programming of the normal air bleed valve, it hasbeen found that when the aircraft is at altitude, typically 25,000 feetor higher, and when the engine undergoes a slow steady statedeceleration corresponding to about 1.5° to 5.0° of power lever movementper second, a surge condition may occur. As noted previously, this surgetypically is manifested by one or two noticeably loud noisesapproximately at location 16 along the steady state line as shown inFIG. 1.

Surge may also occur in the prior art engine as a result of rapid powerlever movements that are not of a sufficient magnitude to cause theengine to pass out of the bleed closed operating area 10. This typicallyoccurs when the pilot reduces power and then nudges the power leverforward to make minor adjustments to the speed of one or more engines.The resultant small but rapid power lever movements are indicatedgraphically by the generally elliptical arrays of arrows 18, 20 and 22.In view of the fact that the latter type of surge typically occurs at ornear maximum compressor speed, there is a substantial probability thatthese surges may cause damage to the compressor.

Turning to FIG. 2, the portion of the subject override system which isdirected to the low energy surges that result from a slow steady statedeceleration is illustrated schematically and identified generally bythe numeral 24. The system 24, is made operative to override the fuelcontrol schedule shown in FIG. 1, and comprises an altitude sensor 26, apower lever position sensor 28 (which is an indirect sensor ofcompressor speed), an electrical power source 30 and a signal overridebleed valve 32. The override valve 32 may either be the bleed valvenormally incorporated into the compressor or a separate valve. Theindirect compressor speed sensor 28 comprises an adjustable cam follower34 and a microswitch 36 which are in communication with the power lever38. In operation, movement of the power lever 38 by the pilot causesrelated changes in both the fuel flow parameters and the core compressorspeed. These pilot actuated movements of the power lever 38 also causethe corresponding movements in the adjustable cam follower 34. When theadjustable cam follower 34 reaches a position corresponding to apreselected angle of the power lever 38 the microswitch 36 is triggered.More particularly, at this preselected position of the power lever 38,the adjustable microswitch 36 is actuated to complete the circuitbetween the electrical power source 30 and the override bleed valve 32.The signal enabled by actuation of the adjustable microswitch 36 causesthe override bleed valve 32 to immediately open, thus venting bleed air31 from the compressor into the ambient surroundings identified asnumeral 33.

The altitude sensor 26 of system 24 is in communication with both theadjustable microswitch 36 and the signal override bleed valve 32. Moreparticularly, the altitude sensor 26 comprises a precision altimeter 40and a lock out switch 42, such that the circuit between the adjustablemicroswitch 36 and the override bleed valve 32 is completed only above apreselected altitude. As a result, regardless of the angular position ofthe power lever 38, system 24 will open override bleed valve 32 onlywhen the aircraft is above an altitude at which surge becomes asignificant problem. In most instances, the altitude sensor 26 isadjusted to complete the circuit at a sensed altitude pressure 27 in therange of 20,000 to 25,000 feet.

The system 24 shown in FIG. 2 substantially reduces the size of theenvelope of operating conditions during which the compressor bleed valveis closed, when the aircraft is at or above the preselected altitude andis shown as a generally shaded triangular area 43 in FIG. 3. Moreparticularly, as shown in FIG. 3, the bleed will stay closed during aslow deceleration only until a core compressor spaced corresponding tothe override bleed open line 44 at point 45 is attained, at which timethe override bleed valve 32 will immediately open to release air fromthe compressor. As illustrated in FIG. 3, the override bleed open line44 at point 45 corresponds to a corrected core compressor speed of about85%. The 85% compressor speed in turn corresponds to a fuel controlpower lever angle of about 54° during a slow steady state decelerationand is shown as line 44. Thus, the override bleed valve 32 will quicklyopen to release air from the compressor at a point prior to theoperating condition under which the deceleration surge would occur in aprior art engine.

To prevent a limit cycle and to provide additional stability, a deadband or hysteresis is incorporated into the indirect compressor speedsensor 28 shown in FIG. 2. This dead band causes the bleed valve toclose during acceleration at a higher compressor speed than the speed atwhich the bleed valve is opened during deceleration. More particularly,with reference to FIG. 3, the dead band in the compressor speed sensor28 effectively establishes an override bleed closed line 46 at which thebleed valve will close during acceleration. Hence during a slow steadystate acceleration the bleed valve will close at a corrected compressorspeed of about 87% which corresponds approximately to a power leverangle of about 58° and is shown as point 48 on line 46. This particularconstruction ensures that the bleed valve does not repeatedly open andclose during a slow acceleration or deceleration.

The system identified generally by the numeral 50 in FIG. 4 is theportion of the subject override system which prevents the high energysurges caused by the small but rapid movements of the power lever ataltitude, such as those identified by lines 18, 20 or 22 in FIG. 1. Itis noted that the system 50 shown in FIG. 4 and the system 24 shown inFIG. 2 can be used independently even though they are typically used incombination. With reference to FIG. 4, the system 50 comprises analtitude sensor 26, an electrical power source 30 and an override bleedvalve 32 identical to like numbered components described above. System50 further includes a rotary potentiometer 52 to convert rotationalmovement of the power lever, into a variable electric signal, plus arate detector/time delay device 54 to measure the period of time overwhich a particular rate of power lever movement occurs. Once a thresholdlevel of rotational motion by the power lever 38 has been detected whilethe aircraft is above a preselected altitude pressure 27, the ratedetector/time delay device 54 will begin measuring the period of timeduring which the power lever 38 is moved at the particular rate. Ifmovement of the power lever 38 continues above the minimum power leverrate for at least a minimum response time, a signal will be sent to theoverride bleed valve 32 causing the override bleed valve 32 to openimmediately and vent bleed air 31 into the ambient surroundings 33. Theoverride bleed valve 32 will remain in its open position for a period oftime which is a function of the specific power lever rate detected bythe rate detector/time delay device 54.

The operation of the override system illustrated in FIG. 4 is showngraphically in FIG. 5. In this example the minimum power lever rate isset to equal approximately plus or minus 3° of power lever movement persecond, as indicated by the vertical dashed line at the left hand sideof the graph. Thus, movements of the power lever 38 at rates less thanplus or minus 3° per second have no effect on the override bleed valve32. However, when the aircraft is above the threshold altitude asdetected by the altitude sensor 26, a movement of the power lever 38 ineither direction at a rate in excess of 3° per second for a sufficientperiod of time will cause the override bleed valve 32 to abruptly open.The duration of sustained movement of the power lever 38 required tocause the override bleed valve 32 to open is illustrated graphically inFIG. 5 by the line identified as "response time". The response timevaries inversely in proportion to the power lever rate. Thus for powerlever rates only slightly above the minimum power lever rate, the ratemust be sustained for a finite period of time, e.g. 0.4 seconds.However, for significantly higher power lever rates, the rate need onlybe sustained for a very short period of time, e.g. at a rate of plus orminus 10° per second, a response time of only 0.1 seconds is required.

The override bleed valve 32 remains open for a period of time whichvaries directly with the rate of power lever movement as illustrated bythe line in FIG. 5 identified as "delay time". Once the override valve32 has been opened, it will remain open for at least a minimum delaytime of 1.0 seconds, as illustrated by the horizontal dashed line inFIG. 5. However, the override bleed valve remains open for aproportionally greater periods at higher rates of power lever movement.Thus, referring to the example cited in the previous paragraph, if apower lever rate of plus or minus 10° per second is sustained for atleast 0.1 seconds, the override bleed valve 32 will abruptly open andremain open for a period of approximately 1.25 seconds. As a furtherexample, if a power lever rate of plus or minus 30° per second issustained for approximately 0.05 seconds or more, the override bleedvalve 32 will abruptly open and remain open for approximately 2.5seconds. The response and delay times of course vary according to theparticular engine in which the subject invention is employed. However,the power lever rate and response time curve is selected to anticipatesurge and the delay time curve is selected to hold the compressor bleedvalve open to release a sufficient amount of air from the compressor andavoid the surge.

In summary, a compressor air bleed override control system is providedto override the primary bleed valve schedule and open the compressorbleed valve quickly when conditions likely to cause the surge ataltitude occur. The apparatus includes an altitude sensor, an electricalpower source, a bleed or override valve in communication with thecompressor, an indirect core compressor speed sensor and a rate detectorfor sensing movement of the power lever. The altitude sensor is adjustedto enable an appropriate signal to be sent to the override bleed valvewhen the aircraft is at or above a preselected altitude. The indirectcore compressor speed sensor is operative to detect a compressor speedwhich corresponds to a slow deceleration. Thus, during a slowdeceleration of the aircraft from altitude, the bleed valve openssuddenly to release air from the compressor prior to the onset of surge.The rate detector is operative to measure both the rate of power levermovement and the duration of movements. When the power lever rateexceeds a selected minimum value and is sustained for at least aselected minimum response time, the bleed valve will open quickly torelease air from the compressor. Furthermore, to ensure that an adequateamount of air is removed from the compressor, the bleed valve remainsopen for a selected period of time which varies according to the rate ofpower lever movement.

While the preferred embodiment of the subject invention has beendescribed and illustrated, it is obvious that various modifications canbe made therein without departing from the spirit of the presentinvention which should be limited only by the scope of the appendedclaims.

What is claimed is:
 1. In an aircraft turbine engine having a multistagecompressor, at least one bleed valve for bleeding air from thecompressor in accordance with a schedule of engine control operatingconditions, and a power lever for controlling fuel flow and compressorspeed, a compressor air bleed override control system for preventingcompressor surge, said override control system comprising:override valvecontrol means for rapidly opening the bleed valve; power lever sensingmeans in communication with said power lever and said override valvecontrol means for sensing power lever parameters that are anticipativeof compressor surge and causing said override valve control means toopen said bleed valve in response to said parameters; and altitudepressure sensing means in communication with said override valve controlmeans and said power lever sensing means for sensing altitude andpreventing said override valve control means from opening said bleedvalve below a preselected altitude, whereby when the aircraft turbineengine is above the preselected altitude the power lever sensing meanssenses power lever parameters that are anticipative of compressor surgeand causes the override valve control means to rapidly open the bleedvalve to release air from the compressor and prevent compressor surge.2. A compressor air bleed override control system as in claim 1 whereinsaid power lever sensing means comprises a means for sensing a selectedpower lever position corresponding to a predetermined compressor speedduring a slow deceleration of the engine, such that when said powerlever reaches the selected power lever position and when the aircraft isat or above the preselected altitude, the means for sensing power leverposition causes the override valve control means to open the bleed valveand release air from the compressor.
 3. A compressor air bleed overridecontrol system as in claim 2 wherein the bleed valve and the overridevalve control means comprise a solenoid valve and wherein the means forsensing a selected power lever position comprises an adjustable switchto vary selection of power lever position in communication with bothsaid power lever and said solenoid valve, said assembly furtherincluding an electrical power source, whereby when the power lever is atthe selected position the switch causes a signal to be sent from saidelectrical power source to said solenoid valve causing air to be bledfrom the compressor.
 4. A compressor air bleed override control systemas in claim 1 wherein said power lever sensing means commprises a meansfor sensing rate and duration of power lever movements, said means forsensing rate and duration of power movements being operative to causethe override valve control means to open the bleed valve when the rateof power lever movement exceeds a preselected minimum power lever ratein either direction for at least a minimum response time.
 5. Acompressor air bleed override control system as in claim 4 wherein saidpower lever sensing means further comprises a bleed time delay means forcausing said override valve control means to close said bleed valveafter a delay period which varies according to the power lever rate. 6.A compressor air bleed override control system as in claim 5 wherein thebleed time delay means is operative to cause the delay period to varydirectly with the power lever rate.
 7. A compressor air bleed overridecontrol system as in claim 5 wherein the means for sensing rate andduration of power lever movement is operative to cause the response timeto vary inversely to the power lever rate.
 8. A compressor air bleedoverride control system as in claim 1 wherein said power lever sensingmeans comprises a means for sensing a selected power lever positioncorresponding to a predetermined compressor speed during a slowdeceleration of the engine and a means for sensing rate and duration ofpower lever movements, whereby when the aircraft turbine engine is at orabove the preselected altitude and when either the power lever reachesthe selected power lever position or when the rate of power levermovement exceeds a preselected minimum power lever rate in eitherdirection for at least a minimum response time, the power lever sensingmeans causes the override valve control means to open the bleed valveand release air from the compressor.
 9. A compressor air bleed overridecontrol system as in claim 1 wherein the means for sensing rate andduration of power lever movements further comprises a bleed time delaymeans for causing said override valve control means to close said bleedvalve after a delay period which varies directly with the power leverrate.
 10. In an aircraft turbine engine having a multistage compressor,at least one bleed valve for bleeding air from the compressor inaccordance with a schedule of engine control operating conditions, and apower lever for controlling fuel flow and compressor speed, a compressorair bleed override control system for preventing compressor surge, saidassembly comprising:override valve control means for rapidly opening thebleed valve; means for sensing a selected power lever positioncorresponding to a predetermined compressor speed during a slowdeceleration of the engine and causing said override valve control meansto open the bleed valve in response to said selected power leverposition; means for sensing rate and duration of power lever movementsand causing the override valve control means to open the bleed valvewhen the rate of power lever movement exceeds a preselected minimumpower lever rate in either direction for at least a minimum responsetime; and altitude sensing means for sensing the altitude of theaircraft turbine engine, said altitude sensing means being incommunication with said override valve control means, said means forsensing a selected power lever position, and said means for sensing rateand duration of power lever movements, said altitude sensing means beingoperative to prevent said override valve control means from opening saidbleed valve below a preselected altitude, whereby when the aircraftturbine engine is at or above the preselected altitude and when eitherthe power lever reaches the selected power lever position or the rate ofpower lever movements exceeds a preselected minimum power lever rate ineither direction for at least a minimum response time, the means forsensing a selected power lever position or the means for sensing rateand duration of power lever movements respectively causes the overridevalve control means to open the bleed valve and release air from thecompressor.